1. Field of the Invention
This invention relates to a handling and cooling system for preforms, made of thermoplastic materials and produced in multi-cavity molds with injection molding machines.
Preforms are cylindrical tubes with one hemispherically closed end and an opposite open end with a ring shaped shoulder and a threaded sleeve. Superficially, they look pretty much like test tubes. Preforms are the initial material for thin wall containers, i.e., bottles for soft drinks, detergents etc., produced with blow molding machines.
Depending on the size of the container to be blow-formed, the preforms have different sizes, particularly in length and wall thickness whereas the diameters are somewhat standardized with the reference diameters being 22, 28 and 38 mm, measured on the outside of the thread.
2. Description of the Prior Art
All production processes for preforms use injection molding machines for transforming the raw material in the shape of firm plastic pellets into a plasticized, deformable melt which is injected, actually pressed, into the cavities of the mold, which determine the shape of the preforms. For the purpose of even melt distribution between the cavities, they are grouped in multiples of four, which means a mold can have from four, eight, sixteen, forty eight up to ninety six cavities.
Typically, and paradoxically the molds themselves consist of three "halves"; the "hot half" and the "cold half", firmly attached to one another and installed on the fixed platen of the injection molding machine. The hot half contains all the heated channels for the distribution of the hot plastic melt to the cavities. The cold half essentially houses the cavities and the cooling channels where cooling liquid, normally chilled water, circulates for cooling the freshly formed preforms at least to the extent that they acquire a temporarily rigid skin, so that they can be released from their cavities without immediate deformation.
The third portion of the mold, installed on the moving platen of the Injection molding machine, accommodates the "mechanical art" of the process with the patterns of the open end of the preform, i.e., ring shaped shoulder and threads, necessitating, for instance, slides for lateral motion facilitating the release of recessed portions of the preforms; furthermore the so-called cores, which form the inner shape of the product.
Ideally, the preforms should sit in their cavities until they are massively cooled down to about ambient air temperature, to prevent any deformation upon their release from the mold. But since the injection molding machine and the mold are the most capital expensive components in the production process of preforms, to allow them to remain in the cavities for such period of time is economically unacceptable. In general, the solution to this problem is to down cool the preforms outside the mold, after their release from the latter. This, in turn, means that the preforms need to be down cooled enough to acquire the above mentioned rigid skin, which allows for adequate time for handling them without causing deformations. Due to residual heat in the preform's material, the rigid surface skin fades away quite fast which again makes further handling impossible. Thus, the post molding process of handling preforms becomes an economical balancing act. It ought not require too much pre-cooling time within the mold, on the one hand, and it must not jeopardize the quality of the preforms, which ultimately are to be bulk packed in boxes for transportation, on the other hand; quality in this case meaning no deformation.
With the above ramifications, it goes without saying that the post mold handling of preforms mandates the use of robotic equipment, mainly and mostly consisting of a linear robot, an end-of-arm tool and a cooling device. While there are various competing systems operating in the preform producing industry, one system has gained wide spread distribution in the global market, because of its high quality output, but it is extremely expensive. At the core of this system we see an end-of-arm tool which is close to identical to three "cold halves" of a mold. They are laterally, firmly attached to each other and can accommodate three production cycles of an injection molding machine. The resulting dwelling time of the preforms in the cavities of the end-of-arm tool, i.e., three times the cycle time of the injection molding machine, plus the fact that these cavities are cooled by circulating coolant, just as in the "cold half" of the mold itself, facilitate the cooling down of the preforms to the point where they can be dropped without peril to quality. The drawbacks of this system are: that it is extremely expensive, that it has a large foot print, that there is relatively slow motion of the end-of-arm tool because of its excessive mass, that it requires a heavy robot to move the end-of-arm tool, and that it requires a relatively long mold intrusion time.